Russ Mitchell wrote:Kel,
I seem to recall last time this came up, it was said that alum wasn't in "real" buff, whatever that is - as opposed to the hot-stuffed, alum-impregnated "hungary tan" hides everybody agreed wasn't real buff leather. Given that, what's substantia and what's accidentia for "real" buff leather?
Short answer - Any bovine leather prepared using the oxidization of marine animal oils. A similar chemical process to that used in the creation of washing leathers called chamois. A similar product can be produced with a blend of oils, wheat gluten and egg yolk.
Kinda slow afternoon so I skimmed through the tannery texts again. Pretty much anything I could find was already summarized in this excellent article written by a former tanner at Clayton: (This won't be news to Russ)
OIL TAN BUFF LEATHER - MANS' FIRST LEATHER
By Mr Joe G Dewhurst , tanner (now retired) at Clayton's Tannery
It is almost certain to say that leather was first made as a result of rubbing oils and greases into skins to try to produce a suitable material for clothing. Skins simply hung or stretched out to dry produce a hard result and which if allowed to get wet will putrefy and disintegrate.
Tanning is the process whereby a chemical reaction between the skin protein (collagen) and a tanning agent (in this case certain types of oils) results in a physical change to produce a material that does not putrefy when wetted. I.e. the skin is converted to leather.
Professor Procter in his Principals of Leather Manufacture (1922 Chapter 26) (1) sums up much of the information produced by other researchers on the subject of oil tannage. ' In its simplest form it consists merely in oiling or greasing the wet skin and kneading and stretching it as it slowly loses moisture and absorbs fat. Under these conditions the fibres become coated with a greasy layer, which prevents their adherence after they are once separated by mechanical treatment. At the same time some chemical change takes place in the fibre itself, which has a part in its conversion to leather varying in importance according to the method and fat employed.
It is perhaps two to three hundred thousand years ago that oil tannage was discovered and slowly evolved by using the most suitable kinds of greasy material. At some stage the use of smoke was found to further improve the leather, both softening and making the leather more water resistant.
Oil tannage using brains and bone marrow originated in Mongolia and it is possible that some 25,000 years ago man crossed the Bering Straits to America taking their hunting and tanning skills with them. The American Indians (as christened by Christopher Columbus) used brains and smoke to tan their skins. The leather, known as Brain Tan, is still produced in America.
L.A.Flemming (2) gives details of the 'Indian Method of Tanning Buckskins.' Clean the flesh side of the skin so that it is free of all fat and muscle tissue, soak to clean and swell and then 'grain' i.e. scrape off the hair and some epidermal layer using a knife, with the skin supported on a wooden beam 6 to8 inches wide. Brains of the animal (deer, pig, horse or cattle) are wrapped in a cloth and boiled in soft water for an hour. Cool to handle and then squeeze until all the brains are mixed in the water. Place the skin in this mixture and knead and stretch, with 10 minute intervals, for an hour. Leave in the mixture for two hours, restretch, leave 1 hour and then hang up until two thirds dry. Then stretch or stake at frequent intervals as the skin is dried out. Allow to age for two weeks by wrapping in another skin to exclude air. It is then ready for smoking by stretching over a smoke tunnel until the smoke penetrates when it is turned over to do the other side. When the desired colour is reached (the smoke causes the yellowish skin to darken) the skin is tightly folded for several days before washing in tepid water. Rinse and hang to dry completely. Leave for a week, dip in warm water for 3 seconds, fold and leave overnight before staking, hanging and restaking to the desired softness. Staking can be done by drawing a blunt blade over the surface in all directions.
An Eskimo method (3) uses a mixture of finely chopped brains and liver from the newly killed animal (reindeer) in warm water to tan the fleshed and shaved skin by kneading and stretching it in the mayonnaise type mixture for half a day. Wring out tightly and hang to dry slowly until almost dry. Strike out in all directions and then stake over a half moon blunt knife until thoroughly soft. It is then ready for smoking over a damp wood fire, being stretched on a circular frame and rotated to give an even effect. Both sides are smoked for about half an hour.
The Inuits also have knowledge of using seal oil, whale oil and smoke.
Furskins can be made using oils applied only to the flesh side, and using the slow drying and stretching techniques described above.
There are rock paintings in Tanzania depicting men and women dressed in leather garments, these have been dated at 35,000 years old. Similar cave paintings in Spain are 20,000 years old. (4)
It is obvious from the above that many oils and fats have been used with at least some degree of success. The tanning power depends on the chemical structure of the constituents of the material. Marine oils are semi-drying, i.e. they oxidize to a sticky glue, but do not dry to a hard film as do drying oils used in paint manufacture. These are the most efficient tanning oils. Oxidation plays a key role in the tanning action.
The most complete tannage system using oil is the 'chamoising' process that is best known for its use as wash leathers. (Presumably the chamois antelope Rupicapra tragus from the high mountains of Europe and Asia, which made particularly soft leather gave the process its name) Cod liver oil is most commonly used because it has the best tanning action, though formerly whale oil and seal oil were used in Europe. Wash leather is made from the flesh splits of sheepskins which have been limed to swell and open up the fibre structure. This process consists of immersing the skins in a solution of lime for up to two weeks or less if the skins are moved in some way. The tanning method is published (1) quite widely and so is not included here in depth, but a brief outline is of interest. The fleshes are delimed with weak acids to remove excess alkali and then well drained. More water is then removed either by pressing, centrifuging or traditionally by stocking with dry sawdust .It is worth mentioning that probably the first mechanisation in tanning was the introduction of milling stocks used to pummel the oil into the skins by hammering the skins which were held in a suitably shaped box or trough. Animal, water, steam and electric power have all been used.(5) The skins become whiter as air is beaten into the structure and water is dried out by the sawdust. Care has to be taken not to allow the temperature to rise too much as a result of the frictional heat generated - that would destroy the skins.
The 'fulling stocks' are used to rapidly distribute the cod oil as it is added to the skins in a series of additions until no more can be taken up. From time to time they have to be taken out and spread to cool. The oxidation of the oil produces heat in addition to that of the mechanical action.
The oxidation (which is responsible for the tanning action) is then completed by packing into boxes where they heat up. Great care is needed to ensure they do not get too hot and gelatinise. They must be taken out to cool and then re-boxed several times until no further heating occurs. I.e. the tannage is complete. A somewhat simpler system was to hang the stocked skins in a warm (100 degrees F.) room for a week or so.
We now have greasy, foul smelling dark brown objects which after degreasing, first by pressure, followed by washing, become the familiar yellow brown wash leather we all know. They are given a ' wheeling' to fluff up the surface and remove any bits of fleshy material.
There is no mention of liming in the older processes discussed above and it is a matter of conjecture as to when the process became known. Liming ( treating the skins by immersing them in a solution of lime) has very beneficial effect on the quality of most leathers and is used extensively in today's tanneries. There are references to the use of wood ash being rubbed in to the fresh skin. Also, naturally occurring salts have been used. This could have led to the discovery of alum 'tanning', another ancient method which still has a place in the leathers produced today. Examples are military accoutrement leather also known as bandsman leather and cricket ball leather. Possibly the production of alum from the shales of North Yorkshire was the first chemical processing to be done on a large scale in England (6)
This was started in 1604 near Ravenscar. Previously alum had to be imported from Turkey at great expense.
Whilst small skins can be penetrated by the oils without liming, it must have been extremely difficult to deal with a cow hide. There is a reference to this in Homer's Iliad where he compares the struggles between Greece and Rome to that of a man trying to stretch a huge cow hide soaked in lard. Evidence of liming dating back to Anglo Saxon times suggests that medieval tanners were using liming to remove hair and open up the fibre structure and would have used this in their manufacture of oil tan buff leather.
Professor Procter (1922) says that buff leather ('much used for military accoutrements') is made in much the same manner as wash leather using ox or cow hides which have had the grain frized off with a sharp knife, after liming thoroughly. The bleaching was done by exposing the damp tanned skins to sunlight. A chemical process used permanganate of potash to make the leather brown, followed by a sulphurous or oxalic acid solution until the colour is removed.
There have been quite a number of tannages used to make a similar type of leather and this has led to confusion. Some of the leathers are made by using a combination of two or more 'tannages'. For instance, the oil tannage may be combined with an alum treatment, or an aldehyde (formaldehyde, glutaraldehyde) treatment may be used. Modern aluminium tannage produces a white leather as does formaldehyde tannage, but the latter is no longer used because of its toxicity. We now have a range of synthetic tannins which also give white leathers. Genuine buff leather is a pale yellow brown colour present through the whole thickness of the skin.
The lack of demand for oil tan buff leather, along with the demise of tanneries in the UK and Europe over the last 50 years or so had led to the disappearance of this leather. However, as Re-enactment Societies became more active, the demand for a genuine period leather returned. It was as a result of requests from Societies and Museums that J Clayton & Sons (Chesterfield) Ltd. used their 140 years of tanning skills to recreate a leather resembling as closely as possible the original oil tan buff.
The demand is for thick leather, usually between 3 to 5mms which requires a starting point using a heavy cattle hide (35 - 45Kgs.) The process has a strong similarity to the chamois process mentioned earlier.
First of all the hide is washed and then treated with a liming solution which burns off the hair and swells up the hide. This is important because the fibre structure is loosened and will more readily absorb the oils.
In order to produce a uniformly thick hide it is necessary to split some of the flesh side off because hides vary in thickness over the area. A band -knife splitting machine is used to do this. Splitting machines were probably the second success in mechanising leather production and first appeared in the late 18th century. The next stage is to remove the strongly alkaline lime residue from the hide.
This is not achieved by using fermenting bran, oatmeal or sour milk as used in the past, but by using acid salts to gently but quickly remove the lime without causing any acid swelling. The pelt is then in the correct condition to receive an enzyme treatment to further open the structure by dissolving out more unwanted proteins. The enzyme preparations used are rather more user friendly than the earlier sources, namely dog dung or pigeon droppings which somehow had been discovered to have a softening effect on leather. Only later was it found to be the natural enzymes present that were the softening agents.
After the appropriate treatment time the enzyme action is stopped by acidifying with strong acid such as sulphuric or hydrochloric and common salt which is necessary to prevent swelling. The acid also contributes to yet more opening of the fibres, which after adjusting to the correct acidity are ready to receive the oiling.
Before the oiling operation begins it is important to reduce the water content by hanging or squeezing through a samming machine. ( like a mangle) If too wet, the oils do not penetrate, if too dry they will not distribute throughout the thickness properly. As the oils go in, the water comes out. The mechanical action required to drive in the oils is produced by tumbling in a wooden drum (7) fitted with shelves so that the leather is lifted and dropped repeatedly. Hot air (40degrees centigrade) is constantly circulated through the drum to reduce the viscosity of the oils and also commence the oxidation process. The mixture that Claytons (8) use consists of cod oil for the tanning, beef tallow as a lubricant and dégras as an aid to penetration. ( Dégras is actually the excess oil which is extracted from wash leather during the cleaning stages) It takes about six hours drumming to get the greases completely through. This is vitally important (and needs skill and experience to determine) otherwise any part not receiving oil will dry hard and boney. The surface colour changes from white to beige during the drumming but the inside remains white and only changes as oxidation proceeds in the next stage.
In this stage the oxidation is encouraged to proceed as quickly as possible by hanging the hides in a hot room maintained at 38 degees C. Fortunately the process can be tracked by observing a new cut edge in the leather - as oxidation proceeds the colour changes. This takes about two weeks and completes the chemical processing. All that remains is to turn the now hard lump of leather into a thick, warm usable leather.
The old method of conditioning and damping followed by much pulling and stretching is carried out. Several modern machine methods are available but at Claytons the drumming method is used. The damp hide is lubricated by the moisture and is readily softened by the tumbling action. Again, the amount of water in the leather is critical, to allow the hide to dry without stiffening. The leather is pulled flat by stapling on to wooden frames during the final drying. The shrinkage that occurs as the moisture is removed pulls the hide taut and flat.
Finally, the leather is tailored to customer requirements of thickness and surface character. A further drumming of the leather will soften it still more, should this be desired.
What would Homer say about our method?
ACKNOWLEDGEMENTS
Thanks to J Clayton and Sons (Chesterfield) Ltd for the assistance in preparing this paper and allowing its publication and to Lawrence Stevens for the invitation to present it on the occasion of their 140th Anniversary.
REFERENCES
1) Procter. Principals of Leather Manufacture. 1922 Chapter 26.
2) Flemming L A Practical Tanning 1910
3) Leather Techniques Through the Ages BASF 75th Anniversary Publication. From a report of the Canadian RCM Police March 1938
4) Leather Techniques Through the Ages BASF 75th Anniversary Publication
5) R Thomson Wolstenholme Lecture Journal of Leather Technologists and Chemists Volume 75 page 85
6) BBC Open2.net Aubrey Manning Landscape Mysteries.
7) The Newcomen Society Excerps from Volume 53 1981 - 82 Patent of George Merchant 1768 Paper by R S Thomson
Joseph Clayton and Sons (Chesterfield) Ltd. Process Document
